1. Field of the Invention
The present invention relates generally to an LCD (Liquid Crystal Display) device and a method for manufacturing the same and, more particularly to, an IPS (In-Phase Switching)-type LCD device having a configuration that liquid crystal is hermetically sealed between a TFT (Thin Film Transistor) substrate and an opposite substrate and also a picture element electrode and a common electrode are arranged on the TFT substrate in such a manner as to be opposite to each other on the same plane.
The present application claims priority of Japanese Patent Application No. 2002-024864 filed on Jan. 31, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
An LCD device is widely used as a monitor and a display device in a variety of information equipment units. The LCD device is made up of a TFT substrate on which is there formed a TFT which operates as a switching element, an opposite substrate, and liquid crystal material which is hermetically sealed between these substrates. Such LCD devices are roughly classified into a TN (Twisted Nematic) type LCD device and an IPS (In-Phase Switching) type LCD device in view of display mechanism.
A TN-type LCD device has such a construction that a plurality of picture element electrodes is arranged on the TFT substrate and a plurality of common electrodes is arranged on an opposite substrate, whereby a driving voltage is applied between the arbitrary picture element electrode and the corresponding common electrode so as to generate an electric field in a direction perpendicular to the TFT substrate and the opposite substrate in operation, so-called perpendicular electric field. The IPS-type LCD device, on the other hand, has such a construction that on one of the two substrates, for example, on a TFT substrate are there arranged a plurality of picture element electrodes and a plurality of common electrodes, each of which is configured to be opposite to the corresponding one of picture element electrodes in a direction parallel to the TFT substrate, whereby a driving voltage is applied between the two electrodes to generate an electric field in a horizontal direction with respect to the substrates in operation, so-called horizontal electric field. Thus, in the case of the IPS-type LCD device, orientation of an LCD molecular occurs along the surface of substrate, for example, the TFT substrate and so, owing to this principle, has an advantage that a wider angle of visibility can be obtained than by the TN-type LCD device. The IPS-type LCD device has become, therefore, used widely and preferably. Such the IPS-type LCD device is disclosed, for example, in Japanese Patent Application Laid-open No. Hei 10-48670.
FIG. 9 shows only one unit picture element of a monochromatic LCD device. FIG. 11, on the other hand, shows only a TFT substrate. As shown in FIGS. 9 to 11, this monochromatic LCD device has a liquid crystal 103 sealed hermetically between a TFT substrate 101 and an opposite substrate 102, in such a configuration that the TFT substrate 101 includes a first transparent insulation substrate 106 made of glass or a like, a first polarizing plate 107 formed on the outer side face of the first transparent insulation substrate 106, a scanning line (gate bus line) 108 which is made of Cr (chrome), Al (aluminum), Mo (molybdenum), or a like and formed on part of the inner side face of the first transparent insulation substrate 106, common electrodes 109 formed on other parts of the surface of the first transparent insulation substrate 106, an inter-layer insulation film 110 which is made of SiO2 (silicon oxide film), SiN (silicon nitride film), or a like and formed in such a manner as to cover the scanning line 108 and the common electrodes 109 in order to provide a gate insulation film partially, a semiconductor layer 113 which is made of an a-Si (amorphous silicon) film or a like and formed above the scanning line 108 via the inter-layer insulation film 110, ohmic layers 113A and 113B which are made of an n+ type a-Si film or a like and formed at both ends of the semiconductor layer 113, a drain electrode 116 and a source electrode 117 which are made of Cr, Al, Mo, or a like and formed in such a manner as to be connected to the ohmic layers 113A and 113B respectively, picture element electrode 121 and a data line 122 which are formed integrally with the drain electrode 116 and the source electrode 117 respectively on the inter-layer insulation film 110, a passivation film (protecting insulation film) 125 which is made of SiO2, SiN, or a like and formed in such a manner as to cover the picture element electrode 121 and the data line 122, and a first oriented film 127 which is made of poly-imide or a like and formed in such a manner as to cover the picture element electrode 121 and the data line 122 via the passivation film 125. In this construction, the scanning line 108, the semiconductor layer 113, the drain electrode 116, and the source electrode 117 are combined to make up a TFT 129. It is to be noted that such a portion of the scanning line 108 as to be present directly below the semiconductor layer 113 acts as a gate electrode. Furthermore, the data line 122 acts as a drain wiring line. Similarly, portions of the common electrode 109 other than such a portion thereof as to be opposite to the picture element electrode 121 serve as a common electrode wiring line 109A.
The opposite substrate 102, on the other hand, includes a second transparent insulation substrate 131 made of glass or a like, a second polarizing plate 133 formed on the outer side face of the second transparent insulation substrate 131 via a conductive layer 132 for preventing electrostatic-electricity, a plurality of black matrix layer regions 134 made of Cr, Ti, or a like, each of which is formed on the inner side face of the second transparent insulation substrate 131, a flattening film 136 formed in such a manner as to cover the black matrix layer regions 134, and a second oriented film 137 which is made of poly-imide or a like and formed on the flattening film 136. Furthermore, a direction arrow indicates a rubbing direction 139 in which rubbing processing is conducted on the first oriented film 127.
This rubbing direction 139 is set as inclined by a constant angle with respect to a longitudinal direction (direction in which the data line 122 is formed) in order to securely determine a twisting direction of the liquid crystal 103 when it is injected into between the TFT substrate 101 and the opposite substrate 102.
The above-mentioned rubbing processing is, as shown in FIG. 12, conducted by revolving a rubbing roller 80 made up of a revolving roller 77 having rubbing hair 79 attached thereto around it via a rubbing cloth 78 so that the TFT substrate 101 as provided thereon with the first oriented film 127 may be moved under the rubbing roller 80, to rub the surface of the first oriented film 127 with the rubbing hair 79, thus forming rubbing trenches.
The following will describe a method for manufacturing a conventional LCD device along steps thereof with reference to FIGS. 13A to 13B through FIG. 16A to 16B. FIGS. 13A, 14A, 15A, and 16A show respective cross-sectional views which correspond to a cross-sectional view taken along line C—C of FIG. 9, while FIGS. 13B, 14B, 15B, and 16B show respective cross-sectional views which correspond to a cross-sectional view taken along line D—D of FIG. 9.
First, as shown in FIGS. 13A and 13B, on the first transparent insulation substrate 106 made of glass or a like, a conductive film which is made of Cr, Al, Mo, or a like and has a film thickness of 200-400 nm is formed throughout the surface by sputtering and then, using a known photolithographic method, is patterned to form the scanning line 108 simultaneously with the common electrode 109 and the common electrode wiring line 109A which are made of this conductive film. The common electrode wiring line 109A, however, is not shown in FIGS. 13A and 13B.
Next, as shown in FIGS. 14A and 14B, the inter-layer insulation film 110 which is made of a SiO2 film, a SiN film, or the like and partially provides a gate insulation film is formed throughout the surface by using a CVD (Chemical Vapor Deposition) method, on which the inter-layer insulation film 110 are sequentially formed the semiconductor layer 113 made of a-Si or the like and the ohmic layers 113A and 113B made of an n+ type a-Si film or the like at both ends of the semiconductor layer 113. Next, a conductive film which is made of Cr, Al, Mo, or the like and has a film thickness of 200-400 nm is formed throughout the surface by sputtering and then is patterned by a known photolithographic method to form the picture element electrode 121, the drain electrode 116, the source electrode 117, and the data lines 122 simultaneously.
Next, as shown in FIGS. 15A and 15B, the passivation film 125 made of a SiO2 film, a SiN film, or the like is formed throughout the surface by using a CVD method and then, the first oriented film 127 made of poly-imide or the like is formed in such a manner as to cover the passivation film 125. Then, as shown in FIG. 12, the rubbing roller 80 is used to conduct rubbing processing on the first oriented film 127.
Next, as shown in FIGS. 16A and 16B, the liquid crystal 103 is hermetically sealed between the TFT substrate 101 and the opposite substrate 102 which is made by sequentially forming a plurality of black matrix layers regions 134 made of Cr, Ti, or the like, the flattening film 136, and the second oriented film 137 made of poly-imide or the like on the inner side face of the second transparent insulation substrate 131 made of glass or the like. Then, the first polarizing plate 107 is formed on the outer side face of the TFT substrate 101 and the second polarizing plate 133 is formed via the conductive layer 132 for preventing electrostatic-electricity on the outer side of the opposite substrate 102, thus completing such the conventional LCD device as shown in FIGS. 9 to 11.
In such a configuration of the above-mentioned LCD device that the main portion of the unit picture element is made up of the common electrode 109 and the picture element electrode 121 which are opposite to each other in a comb-teeth shape, the common electrode 109 and the picture element electrode 121 are formed at the same time as the other electrodes or the wiring lines are formed, from a viewpoint of an advantage in manufacturing method. That is, as described above, the common electrode 109 is formed of the same conductive material as the scanning line 108 and the common electrode wiring line 109A and at the same time as they are formed. Furthermore, the picture element electrode 121 is formed of the same material as the drain electrode 116, the source electrode 117, and the data line 122 at the same time as they are formed. Accordingly, it is possible to eliminate a manufacturing process of forming the electrode or the wiring line, thus avoiding an increase in costs.
In the conventional LCD device and method for manufacturing the same, however, since a common electrode 109 and a picture element electrode 121 which make up the main portion of a unit picture element are formed to have a relatively thick film, a step occurs on the first oriented film 127 formed on a protecting insulation film via these two electrodes, so that rubbing processing so as to form the first oriented film 127 cannot be sufficiently performed, which is a problem.
That is, in the conventional LCD device and the method for manufacturing the same, as described above, although the common electrode 109 and the picture element electrode 121 which make up the main portion of a unit picture element are both formed of the same conductive material as the other electrodes or wiring lines and at the same time as they are formed, two kinds of electrodes, the common electrode 109 and the picture element electrode 121 are formed to have a relatively thick film, so that a step 140 occurs in the first oriented film 127 as shown in FIG. 10 when the first oriented film 127 is formed via the passivation film 125 on these two electrodes, the common electrode 109 and the picture element electrode 121 and, therefore, such rubbing processing as shown in FIG. 12 cannot sufficiently be conducted because it is interfered by the step 140.
The above-mentioned LCD device, which is widely used as a monitor of medical-care equipment, has a performance requirement of, especially, a high contrast for this type of monitor. To satisfy this requirement, it is important in configuration of the LCD device to conduct rubbing processing sufficiently on the first oriented film 127 formed on the TFT substrate 101 in order to improve the orientation of the liquid crystal 103. On a conventional LCD device, however, the rubbing processing cannot sufficiently be conducted for the reason described above, so that a high contrast has been difficult to achieve.
It is to be noted that the film thickness of the common electrode 109 and the picture element electrode 121 is determined by the required film thickness conditions of the common electrode wiring line 109A which is formed simultaneously with these two electrodes, the common electrode 109 and the picture element electrode 121, or of the data line 122. That is, if the LCD device is viewed as a display product, it must satisfy a basic concept of saving on power dissipation as much as possible; so that, in such the LCD device as described above in which a number of unit picture elements are integrated to make up the product, it is necessary to reduce the wiring line resistance of the data line 122 and the common electrode wiring line 109A which supplies power to the common electrode 109 and the picture element electrode 121 which make up the main portion of the unit picture element. To do so, it is in turn necessary to increase the film thickness of these two wiring lines, the common electrode wiring line 109A and the data line 122, which results in formation of a relatively thick film of the two electrodes, the common electrode 109 and the picture element electrode 121, as well which are formed simultaneously. The common electrode 109 and the picture element electrode 121 themselves function sufficiently even if they are relatively thin and so need not be formed as thick as the common electrode wiring line 109A and the data line 122.
Therefore, to decrease the size of the step 140 in the oriented film 127 which is formed on both of the electrodes, the common electrode 109 and the picture element electrode 121, it is necessary to form a conductive material as a relatively thin film, which has been, however, difficult in a conventional LCD device and by a method for manufacturing the same. If the rubbing processing cannot sufficiently be conducted as in the case of the conventional method, the contrast is deteriorated drastically, especially in a normal-black LCD device.